The present invention relates to roofing structures for buildings, and more particularly to fire retardants for roofing structures which utilize thermoplastic insulation.
Roofing structures for large commercial buildings typically utilize fluted metal decks of steel or aluminum. The metal decks are usually overlain with one or more layers of insulation, waterproofing material, and ballast material. Many types of insulation materials are used in roofing structures. One type of insulation material which is used widely is thermoplastic foam. Thermoplastic foam insulation materials are used widely because they are relatively light weight and have superior insulative properties.
One difficulty encountered with the use of thermoplastic foam insulation in roofing structures is that thermoplastic foams can melt and burn, thereby contributing to a fire. For example, molten plastic insulation can contribute to a fire by internally self-propagating the spread of fire in a roof deck. Internal self-propagation of fire is a condition wherein fire spreads inside the roofing assembly, after the roofing material is ignited by the heat from a fire within a building.
Standards for roof construction were established to prevent this type of fire after a fire occurred at a General Motors plant in Livonia, Michigan. This fire resulted in a $35,000,000 loss and the total collapse of the 30-acre structure. Due to the nature of the plant's roof construction, hot, combustible gases were unable to escape the roofing assembly and subsequently contributed to the fire directly below the roof structure.
As a result, building codes specify fire spread performance criteria for roofing structures. These criteria are determined by nationally recognized test standards for building assemblies. For example, some building codes require that a 15-minute fire or thermal barrier be incorporated in a roof assembly between foamed plastic insulation and occupied interiors unless the roof construction has passed a diversified test such as a test conducted by Underwriters Laboratories, Inc. The UL test utilizes a test structure on which a roof assembly is constructed which is 20 feet wide by 100 feet long and 10 feet high. A fire is started at one end of the structure to determine the burning characteristics of the test structure. The determination of whether the test structure passes the UL test is made by comparing the performance of the test structure to the performance of a "standard" roof structure utilizing a one-inch vegetable fiberboard insulation, which is mechanically affixed to the steel deck and overlain by a asphaltic, built up membrane. In order for the test structure to pass the test, underdeck flaming must not exceed 60 feet, with tips of the flaming not extending beyond 72 feet from the end of the structure at which the fire is started.
Various methods of roof construction have been proposed to reduce the likelihood that plastic foam insulation will contribute to a fire. For example, Hyde et al. U.S. Pat. No. 3,763,614: Curtis U.S. Pat. No. 3,466,222: and Kelly U.S. Pat. No. 4,449,336 are representative of one type of solution. Hyde, Curtis and Kelly attempt to solve the aforementioned problem by interposing a non-combustible material between a metal roof and a layer of thermoplastic foam.
In Hyde et al, a metal deck is overlain with a non-combustible insulating layer comprised of gypsum board, foamed glass, ceramic foam, or thermosetting plastic foam. A water impermeable layer overlays the non-combustible layer, and a thermal insulating layer overlays the water impermeable layer. A protective surface comprised of gravel or sand and cement is placed over the thermal insulating layer.
Curtis relates to a fire retardant structure utilizing an insulative laminate. Curtis laminate includes a lower foil layer, which is overlain by a lamina formed of at least 50% unexpanded vermiculite in a binder. A foam core is disposed above the lamina and an upper traffic and mopping surface overlays the plastic foam insulation layer.
Kelly relates to a roof structure wherein a metal deck is overlain by a fireproof member which is preferably made of plaster board. A reservoir board overlays the fireproof member and includes a plurality of apertures. The reservoir board is preferably formed of gypsum, fiberboard, or Perlite. A layer of insulation overlays the reservoir board. In a fire hot enough to melt the insulation layer, the molten insulation is captured in the apertures of the reservoir board.
Richards et al, U.S. Pat. No. 4,073,997, relates to another type of proposed solution of the aforementioned problem. Richards discloses a composite panel which includes an organic foam core which is sandwiched between two layers of inorganic fibers.
Although the systems proposed in the above-discussed patents do serve to reduce the flammability of thermoplastic insulation, the addition of a non-combustible layer between the deck and the insulation adds significantly to the cost of a roofing structure. This additional cost can place the use of plastic insulation at a cost disadvantage.
Another solution was proposed by the Working Group Concerned with Roofs in the West German Fire Protection Association in an article entitled "Fire Safety and Thermally Insulated Flat Roofs with Trapazoidal Steel Profiles--Parts I and II: Final Report". 1986 Fire Safety Journal, No. 10, pages 139-147 (originally published in the German language in VFDB-Zeitschrift 33 (2) (1984) 44-49 and 50-53). One of the solutions proposed in the Working Group report involves the placement of fire stops in the grooves of the metal deck. These fire stops are provided to block the flow of gases or liquids given off by the melting insulation into the building. Preferably, these fire stops should be non-combustible and should reliably block the cavities at temperatures of about 800.degree. C. The materials used for forming the fire stops should also be sufficiently dense to prevent the passage of gaseous and liquid products of decomposition. The materials must also adequately withstand the mechanical loads acting on the roof under normal thermal and load conditions.
Although the Working Group report does disclose an alternative to the interposition of a non-combustible layer between a metal deck and a thermoplastic insulator layer, room for improvement exists.